Optimization of UAV-mounted Intelligent Reflecting Surface for the Downlink of Cellular Systems

Intelligent reflecting surface (IRS) recently appears as a promising technology for the next generation wireless network owing to both spectral and energy efficiency enhancement capability. On the other hand, unmanned aerial vehicle (UAV) technology has received increasing interest in many applications from the wireless community. In this paper, we study a downlink (DL) communication network, wherein single-antenna users (UEs) are served by a multiple-antenna base station (BS). To assist the communication between UEs and BS under the assumption of no direct link from BS to UEs, we propose to mount the IRS on the UAV platform in order to exploit the advantages of both UAV and IRS technologies. In particular, the BS beamforming, the IRS phase shift and the UAV placement are jointly optimized to maximize the sum rate performance. Owing to the high coupling properties of the optimization variables, particularly the beamforming and IRS phase shift variables, the optimization problem is naturally non-convex. To effectively solve it, we propose an iterative algorithm by utilizing block coordinate and inner approximation methods. Numerical results are used to show that the proposed approach can significantly improve the system sum rate performance.